Recently, in accordance with development in electronic technology, a number of portable electron devices such as VTRs (video tape recorder) with a built-in camera, cellular phones and laptop computers have come into wide use, and miniaturization and weight-reduction of the devices have become the subject. Research and developing aimed at improving energy density of batteries used for portable power sources for the devices, specially secondary batteries, have been actively conducted.
Widely known secondary batteries of the related art are a lead battery, a Ni(nickel)-Cd(cadmium) battery, a lithium ion secondary battery in which a material such as a carbonaceous material capable of occluding/releasing lithium (Li) is used for a negative electrode, and a lithium secondary battery in which lithium metal is used for a negative electrode. A large expectation has been put on a secondary battery using a non-aqueous electrolyte, specifically on a lithium ion secondary battery, since the battery can obtain a higher energy density than the lead battery and the nickel-cadmium battery of the related art using an aqueous electrolyte, and its market has largely grown. Also, theoretical electrochemical equivalent of lithium metal in the lithium secondary battery is as large as 2054 mAh/dm3, which is equivalent to 2.5 times the graphite material used in a lithium ion secondary battery. Thereby, an excellent energy density higher than that of the lithium ion secondary battery can be expected and the lithium secondary battery has been actively studied.
However, while being capable of obtaining a large capacity, a lithium secondary battery has low charging/discharging efficiency. Also, it has a problem such as deterioration in the charging/discharging capacity when repeating charging and discharging, thereby having an insufficient charging/discharging cycle characteristic. Especially, the problem is noticeable when performing boosting charge by a large current for a short time. Therefore, it is difficult to perform boosting charge on the lithium secondary battery. Also, in the lithium secondary battery, lithium is consumed by repeating charging and discharging. Therefore, it is necessary for the lithium secondary battery to contain an excessive amount of lithium in advance. As a result, there is another problem such that the actual charging/discharging capacity cannot be made much larger.
The problems are directly due to the fact that lithium metal forming a negative electrode is pulverized during a dissolving/re-crystallizing process of lithium metal at the time of charging/discharging. As techniques for suppressing the pulverization, a variety of improvement methods are proposed as noted in “Lithium Batteries” (Edited by JEAN-PAUL GABANO, ACADEMIC PRESS, 1983, London, New York). For example, a lithium alloy such as a lithium-aluminum alloy is used as a negative electrode material, a variety of additives are added to an electrolyte, or the surface of the lithium metal is slightly coated by a carbonaceous material. However, none of these methods are sufficient and it has been still difficult to put a lithium secondary battery in a practical use.
One of the reasons for having difficulties suppressing pulverization of lithium metal is that the volume of the negative electrode made of lithium metal largely changes at the time of charging/discharging. For example, as may be evident in the characteristic of manganese-lithium secondary batteries on the market, the amount of displacement is small in the distance between the positive electrode and negative electrode when charging/discharging is shallow, and the electrode reaction on the surface of the negative electrode tends to proceed homogeneously. On the contrary, when charging/discharging is deep, the amount of displacement is large in the distance between the positive electrode and negative electrode and the displacement phenomenon tends to proceed heterogeneously. Therefore, the distance between the positive electrode and negative electrode tends to be heterogeneous. These can be considered as the reasons for promoting pulverization of lithium metal at the time of charging/discharging.
According to the hypothesis, it is assumed that pulverization of the lithium metal can be suppressed by minimizing the displacement in the distance between the positive electrode and negative electrode as small as possible so that the charging/discharging cycle characteristic can be improved.
For example, one of the methods for minimizing the amount of displacement in the distance between the positive electrode and negative electrode is minimizing the amount of reaction of the negative electrode made of lithium metal. However, if this method is applied to a lithium secondary battery of the related art in which lithium metal is provided on a collector layer, the energy density of the battery is largely deteriorated. As a result, the significance of using lithium metal for a negative electrode, which initially has high electrochemical equivalent, becomes insignificant.
Another method for minimizing the amount of displacement in the distance between the positive electrode and negative electrode is to maintain the distance between the positive electrode and negative electrode constant by applying a pressure through providing a spring or the like. However, if a spring or the like with no electrode activity is provided inside the battery, the fraction of the volume of the electrode active material inside the battery becomes relatively low for the volume of the spring or the like provided. Thereby, the discharging density and the energy density of the battery deteriorates.
Therefore, development of a secondary battery in which the amount of displacement in the distance between the positive electrode and negative electrode can be minimized without deteriorating the characteristic capable of obtaining a high energy density has been demanded. In developing such a secondary battery, it is necessary to study the composition of an electrolyte in order to sufficiently utilize the capacity of the electrode in addition to studying the electrode material.
The invention has been designed to overcome the foregoing problems. The object of the invention is to provide a secondary battery in which a high energy density can be obtained and the charging/discharging cycle characteristic can be improved.